This invention relates to scanning probe microscopy, such as for example, atomic force microscopy or near-field optical microscopy, and more particularly to a fast translation stage for scanning probe imaging that avoids imaging distortion associated with rapid changes of scan direction.
It is highly desirable to increase the speed with which scanning probe microscopes can image, particularly in fluid, in order to permit visualization of processes that occur on timescales comparable to or faster than the imaging rate of conventional scanning probe microscopes. Present commercially-available scanning probe microscopes are limited by natural mechanical resonances so that several seconds or more are required to acquire an image. It would be highly desirable to be able to increase these imaging rates by an order of magnitude or more. In such a case, ten or many tens of images per second could be acquired, giving rise to near-video rate data acquisition.
One of the limiting resonances is associated with the force sensing cantilever itself. The resonant frequency, f0, of a cantilever of effective bending force constant K in Newtons per meter (N/m) and mass m in kilograms (Kg) is given by
      f    0    =            1              2        ⁢        π              ⁢                            K          m                    .      Because it is desirable to keep the force constant, K, small for imaging soft materials, the route to increasing the resonant frequency lies in making smaller cantilevers so as to reduce their mass m.
Viani et al, “Fast imaging and fast force spectroscopy of single biopolymers with a new atomic force microscope designed for small cantilevers,” Rev. Sci. Instrum. 70: 4300-4303 (1999), and Hansma et al, U.S. Pat. No. 5,825,020, describe microscopes in which the length of the cantilever is reduced to only a few times the optical wavelength, so that, using special focusing optics as taught in the '020 patent, cantilevers with resonant frequencies of greater than 100 kHz (in water) and with spring constants as small as 0.06 (N/m) can be used. Ando et al, “A high-speed atomic force microscope for studying biological macromolecules,” Proc. Natl. Acad. Sci. (USA) 98: 12468-12472 (2001), take a similar approach and have reported imaging at greater than 10 frames per second with cantilevers having resonant frequencies of about 500 kHz (in water) and with spring constants of 0.15 to 0.28 N/m.
In practice, another severe limit on scanning speed is dictated by the sample (or probe) scanning stage. In order to fully exploit a cantilever with a 500 kHz resonant frequency, each line scan, sampling typically 100 or more points, would have to be completed in (5 kHz)−1 seconds or 20 microseconds (μs) if a height data point is to be acquired on each cycle of cantilever oscillation.
Current practice in scanning probe microscopy includes scanning the probe in a raster pattern over the sample as shown in FIG. 1A. Alternately, the sample may be scanned in a raster pattern under a fixed probe. Considering just the fast scan axis (taken to be the X axis here) and referring to FIG. 1A, a time-varying voltage, V(t) 1, is applied to electrodes 2, 3 on a piezoelectrically driven scanning element 4 so as to cause the element to bend and scan a probe 5 over the surface of a sample 6. The desired motion is a linear sweep of the probe over the surface so that the displacement X(t) varies with time as shown in FIG. 1B.
Because the displacement of a piezoelectric material is not linear with changes of applied voltage, it is usual to apply a non-linear ramp, such as that shown as V(t) in FIG. 1C so that the resultant displacement, X(t), is a linear function of time. See, Eilings, V. B. and J. A. Gurley, U.S. Pat. No. 5,051,646, and Lindsay, S. M. and I. W. Shepherd, “Linear Scanning Circuit for a Piezeoelectrically Controlled Fabry-Perot Etalon,” Rev. Sci. Instrum. 48: 1228-1229 (1977). This approach results in a rapid change of direction at the turning points 7 of the scan where the derivative of the applied voltage changes sign as shown at 8.
Because the velocity of the probe is proportional to the time derivative of the applied voltage,
            ⅆ              V        ⁡                  (          t          )                            ⅆ      t        ,the momentum change, or impulse applied to the scanning element is proportional to the second derivative
            ⅆ                                                 2                    ⁢          V                ⁡                  (          t          )                            ⅆ              t        2              .This quantity is infinite at the turning points 7 and 8, though in practice it is limited by the time response of the driving electronics. The result is that a sharp impulse is given to the scanning element at the turning points, and this causes the scanning element to ring at its resonant frequency, f0SCANNER.
The ringing continues for approximately Q/f0SCANNER seconds, where Q is the mechanical Q factor of the scanning element. Because a rapid response is desired from the scanning element, and critical damping is not easy to implement, scanning elements generally have a Q>1. Thus, given a typical scanning element with a resonant frequency of a few kHz and a Q of 5, the ringing motion, 9 in FIG. 1D may continue for 5 or more milliseconds. If, in turn, this distortion is not to affect more than 10% of a scan, the fast scan time is limited to 50 milliseconds or more, which is several hundred times slower than needed to realize the potential speed offered by small (optical wavelength-sized) cantilevers.
One solution to this problem has been proposed by Ando et al, supra, using a balanced pair of scanning elements moving in opposite directions, one scanning the sample stage, and the other scanning a dummy mass. The scanning elements are driven so that the total momentum of the system is always approximately zero. This, however, greatly increases the mass and complexity of the scanning stage and also increases the possibility of spurious resonances in the scanning element.
Accordingly, a need still exists in the art to provide a fast-scanning stage that is free from artifacts associated with any turn-around in scan direction. There is a further need for a scanning stage that can complete fast line scans at a rate of several kHz, and which is free from turn around artifacts. There is a further need in the art for a scanning stage having scan amplitudes on the order of approximately one micron with small (i.e., less than about one hundred volts) voltages applied to the scanning elements.